To succeed in golf, a golfer must hit a ball a long distance in the right direction. By the 1990s, it had become accepted wisdom that a wood-type club with a large, hollow head was the best tool for that job. The idea was that an oversized head gives the club a large sweet spot, which helps the ball fly a long distance in the right direction. A typical driver head has a minimal surface area per volume and has no substantial concavities or indentations. Designers believe that such designs give the best compromise on sweet spot, aerodynamics, and mass distribution.
Typical club heads are made of layered composite materials or metals that must be at least a certain thickness (e.g., at least six layers of composite) for strength. A club head designer that wants to free up discretionary mass will bring the thickness of the club head walls down to the minimum while simultaneously minimizing the surface-area-to-volume within allowable design constraints. The idea has been that thinning walls down to the structural limit and minimizing surface area frees up the most possible discretionary mass.